The present invention relates to an electronic component mounting method implemented in an automatic electronic component mounting apparatus.
Electronic component mounting apparatuses have been desired to be operated at a high speed and with high accuracy in recent years. A multi-functional component mounter equipped with a component transfer head movable in X and Y directions by means of an X-Y robot for picking up and mounting electronic components has been accordingly widespread in the market because of the flexibility of component supply modes, the abundance of component types that can be mounted, and high mounting precision due to simplicity of its construction.
At the moment, in an attempt to further accelerate the operation speed and to achieve even higher precision in the electronic component mounting apparatus, there are being developed X-Y robots which employ a linear motor, or component suction nozzles with an upward/downward drive mechanism which can be pressure controlled so as to adjust pressure when mounting components. For high-speed operation of such nozzles, there have been developed an upward/downward drive mechanism having a drive source, such as a rotary motor or air cylinder, and a ball screw or link mechanism combined with such drive source, or a voice coil motor (hereinafter referred to as VCM) having a shaft in the interior thereof, at the lower end of which a nozzle is attached uniformly or in operative connection so that the nozzle is directly driven upwards and downwards at a high speed.
One conventional electronic component mounting apparatus using this VCM will be described with reference to FIG. 5 showing its system construction and FIG. 6 which is a flow chart of control actions in the event of an abnormal condition.
In FIG. 5, 1 is a nozzle for picking up and mounting electronic components, 2 is a VCM having a shaft 8 for causing the nozzle 1 to move upwards and downwards, 3 is a position detector or a magnet scale for detecting the position of the VCM 2 in upward and downward directions, 4 is a VCM control unit for executing positioning control of the VCM 2 and torque control of same by varying the amount of electric current supplied to the VCM 2, 5 is an input/output control unit for controlling suction/evacuation of the nozzle 1 and actions of a gripping device, 6 is a gripping device for preventing the fall of the nozzle 1 by holding the shaft 8, and 7 is a system control unit for sending the positioning control instructions and torque control to the VCM control unit 4.
Control of the electronic component apparatus described above will be explained with reference to FIG. 5. VCM 2 is controlled by a servo loop including magnet scale 3 and VCM control unit 4. In a servo lock state, the gripping device 6 is opened in accordance with the instruction from input/output control unit 5. When performing picking up and mounting operations of electronic components, the VCM control unit 4 drives the VCM 2 towards a predetermined position in accordance with instructions from system control unit 7, whereby the positioning of the nozzle 1 is achieved at a high speed.
In case the magnet scale 3, which performs position detection of the VCM 2, is disconnected and thereby positioning control by the VCM control unit 4 is impeded, control is effected as follows under order to keep the VCM 2 in control. Referring to FIG. 6, when disconnection is detected at step #1, the VCM control unit 4 outputs a braking signal to the input/output control unit 5, upon which the input/output control unit 5 instructs the gripping device 6 to close at step #2, and at the same time (at step #3), the VCM control unit 4 cuts a drive power supply to the VCM 2. By the above control, the upper end of the shaft 8 which is driven upwards and downwards within the VCM 2 is grasped by the gripping device 6, and the nozzle 1 attached at the lower end of the shaft 8 is prevented from falling.
However, in the control method of electronic component mounting apparatus as described above, there exists a time lag between the points when the drive power supply to the VCM 2 is cut and when the gripping device 6 holds the shaft 8. Since the sliding resistance between the shaft 8 driven upwards and downwards and the gripping device 6 is set as small as possible in this type of electronic component mounting apparatus because of the characteristics of VCM 2, there is a problem that the nozzle 1 at the lower end of the shaft 8 free-falls during this time lag. This problem occurs whenever the positioning control of the VCM 2 is impeded by any failures such as power outage of the VCM control unit 4, other than the case where the magnet scale 3 is disconnected.